Photolithography, the art of using light to transfer an image from a master pattern to another medium, is fundamental in the manufacture of semiconductor and electro-optical devices. In a typical photolithographic operation, a workpiece such as a planar semiconductor substrate is coated with a photosensitive, etch-resistant material called photoresist. The photoresist is then exposed to a pattern of activating radiation by transmitting light through a patterned mask onto the coated surface. After exposure, the resist is developed. If the resist is "positive-working", the exposed resist is washed away. Typically small openings, called windows, are opened in the resist permitting selective treatment, such as etching or doping of the exposed substrate. Thus precise two dimensional patterns can be defined and treated.
Photolithographic etching operations are typically "binary" operations in the sense that they are typically used to either remove a given two dimensional area of a layer or the area is not to be etched. In the usual case a single photolithographic etching step provides only limited ability to shape the topology of a workpiece surface. Significant topology shaping typically requires special etchants and numerous etching steps with different masks.